A siren of the general type to which the present invention relates is mounted at a fixed location, often on top of a building or on a tower, so that its sound output will be heard at substantial distances. In most cases it must be omnidirectional, that is, it must emit sound substantially uniformly in all directions so that it can be heard in all parts of a community that it serves.
In general, a siren of the type here under consideration comprises a sound generator consisting of a radially apertured rotor or so-called chopper that is concentrically rotatable in a radially apertured casing or stator. The rotor, which is usually driven by an electric motor, serves, in effect, as a pump that draws in air and pressurizes it. As the rotor turns, its apertures move through transient register with the apertures in the stator to permit intermittent escape of the pressurized air for sound generation.
Associated with the sound generator is a sound directing means which prevents the sound energy from being dispersed upwardly. This may take the form of a system of louvers around the sound generator, or, as is more common, the sound generator may be mounted in a closed end portion of a lengthwise flaring horn or megaphone that concentrates the sound energy into a more or less directional beam. When a siren comprising a horn is required to produce an omnidirectional signal, the horn is usually caused to rotate about a vertical axis during operation of the siren, at a rate on the order of a few revolutions per minute. Heretofore an omnidirectional siren comprising a rotating horn has almost invariably had a collector ring connection through which its motor was energized, to accommodate rotation of the horn. Such a connection was not completely dependable, because wear and accumulations of foreign matter could break the energizing circuit.
A more serious problem, heretofore encountered with sirens installed in cold weather climates, has been lock-up of the siren by ice frozen between the stator and the rotor of the sound generator. Almost all prior sirens have been so arranged that rain, snow or sleet could enter the sound generator, either being blown directly into it by the wind or being drawn into it through its air inlet when the siren was operated during inclement weather. Although snow or sleet, as such, would not materially interfere with operation of the siren, such frozen precipitate could be melted in the sound generator as a result of air compression that heated the device during siren operation. Thus, moisture that entered the sound generator in any form could freeze during cold weather and form a solid bridge between the stator and the rotor whereby the latter was locked against rotation.
Sirens have been known and used in emergency warning systems for decades, but the above explained problem of freeze-up has persisted without a satisfactory solution. Everyone concerned with the art has certainly recognized that a siren, as an emergency warning device, must be absolutely reliable under all conditions. It is evident, therefore, that something beyond mere skill in the art has been needed for satisfactory solution of the problem.
Two prior patents have disclosed arrangements that were intended to prevent weather related failures of sound-signal warning devices, but each of them was markedly unsatisfactory in certain respects.
U.S. Pat. No. 1,323,826, issued to O. S. Burke in 1919, disclosed a siren having a dome-shaped or approximately spherical housing which surrounded the sound generator with a rather large clearance and which was mounted on a base plate for rotation about a vertical axis. The housing had a relatively small concentric opening in its top through which its interior was communicated with the narrow end of a horn that was fixed to the housing and extended upwardly a short distance from it, then curved around to project laterally a substantial distance to one side of the housing axis. Vanes on the inside of the housing were arranged to be acted upon by air discharged from the sound generator for rotating the housing about its axis, to thus swing the horn around in a circle. Because the housing necessarily converged towards its top outlet to provide for such air-driven rotation, the acoustic efficiency of the arrangement was very low. Furthermore, the vanes on the inside of the housing, acted upon by air expelled from the sound generator, provided a somewhat impositive drive for horn rotation whereby the weathervane effect of the wind on the long, laterally extending horn could cause the horn to turn at a rate that varied markedly from point to point around its orbit, or could prevent it from rotating if the wind were strong enough. Furthermore, there was a possibility of the sound generator overheating because air discharged from it tended to be recirculated back to its inlet by the dome-like shape and the restricted top outlet of the housing.
U.S. Pat. No. 2,198,026, issued to C. C. Farmer in 1940, disclosed a locomotive steam whistle that was rotatable through around an upright axis. The whistle comprised a horizontally extending horn having a sound generator mounted at its small diameter end. In its normal position the mouth of the horn opened rearwardly, to prevent accumulations of snow, dirt and the like from being driven into the horn by forward movement of the locomotive and to direct signals to train crew members in the caboose. For forwardly projected signals the horn was swung around to its opposite position by a pressure responsive actuating device operating under the control of solenoid valves connected with a selector switch in the locomotive cab. The patent evidenced recognition of the problem posed by an intrusion of foreign matter into a device that produces a sound signal, although the arrangement that it disclosed was obviously unsuitable for an omnidirectional community warning siren.